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Usually called theories of illness, these are the concepts people use to explain what they perceive to be wrong with their health. Ancient theories include the influence of evil spirits, witchcraft, the wrath of a vengeful god. Ancient Greeks and medieval Europeans held a humoral theory, that character was determined by the balance among four “humors”—phlegm (phlegmatic), choler, or yellow bile (choleric), blood (sanguine), melancholy, or black bile (melancholic)—and that many diseases arose because of imbalances among these humors. The theory and concept of contagion is ancient: it was epitomized in stigmatizing of lepers and others with disfiguring skin diseases. A superstitious belief in the contagiousness of cancer and mental disorders hindered advances in their investigation and care until the 1800s. The miasma theory, that some diseases were caused by an ill-defined emanation from rotting organic matter, persisted into the mid-19th century. The miasma theory was supported by the observed association of malaria with swamps. The germ theory has ancient origins but did not become dominant until the discovery of microscopic organisms. It is abundantly confirmed when the Henle-Koch postulates are fulfilled. Scientific advances since the late 19th century have identified many biochemical, metabolic, and hormonal causes of disease. The theories of Sigmund Freud and others seek to explain mental, emotional, and personality disorders and some physiological manifestations of disease on the basis of tensions between subconscious and conscious thought. Some important diseases, such as multiple sclerosis, remain unexplained.